Field of the Invention
The present invention relates generally to a method for protecting a layer, including a method for capping a doped silicate glass layer to block diffusion of dopants, by forming a high-quality non-doped silicate glass layer by plasma-enhanced atomic layer deposition (PEALD).
Related Art
U.S. Patent Application Publication No. 2013/0115763 discloses methods for forming doped oxide thin films such as borosilicate glass (BSG), phosphosilicate glass (PSG), and arsenic silicate glass (ASG), the disclosure of which is herein incorporated by reference in its entirety, for forming a doped silicate glass layer. As described in the publication, solid state diffusion (SSD) is a technology to diffuse dopants such as phosphorus, boron, and arsenic in a silicon substrate by heat treatment of a doped thin film (e.g., PSG, BSG, ASG) containing the dopants formed on the substrate. One problem in SSD is that dopants vaporize and dissipate from the doped thin film with time elapsing after formation of the thin film and during the heat treatment, rather than dispersing into the silicon substrate, resulting in insufficient diffusion of the dopants in the substrate. Conventionally, in order to manage the above problem, a non-doped thin film (NSG) containing no dopant is formed on the doped thin film to inhibit dissipation of the dopants from the doped thin film.
As discussed above, dopants are likely to dissipate from a doped thin film before heat treatment for SSD, i.e., during storage (exposed to the atmosphere), and during the heat treatment. In order to inhibit dissipation of the dopants from the doped-thin film, a cap film made of non-doped silicon oxide or non-doped silicate glass (NSG) can be formed on the doped thin film. However, a desired level of dispersion of the dopants in the substrate has not been achieved in SSD. If the thickness of the cap film is increased, the cap film is expected to improve blocking dissipation of the dopants so as to increase the concentration of dopants dispersed or diffused in the substrate; however, a thick cap film is not suitable for processes which require thin layering.
The quality of a cap film (e.g., high resistance against wet etching) can be improved by densification of the film, but such a process likely causes oxidation of the underlying layer or the substrate.
The above discussion of problems and solutions and any other discussions disclosed in this disclosure in relation to the related art has been included solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion was known at the time the invention was made.